Heart: contractility v.s force of contraction

They're different, but they're similar enough that you'll probably never need to know the difference. An increase in one usually goes hand-in-hand with an increase in the other, and even some physiology textbooks will use the two interchangeably.

Contractility is the ability for the myocardium to contract. Force of contraction is the amount of force that the myocardium contracts with.